Acoustic microscopy is a relatively recent technological field which was developed in the early 70's. The theory of scanning objects with ultrasonic waves is relatively simple and consists of applying microwave pulses of very short duration to a piezoelectric transducer mounted on a lens made usually of saphire or quartz. The ultrasonic waves emitted by the transducer propagate to a spherical cavity ground in the opposite face of the lens. The cavity is filled with liquid, usually water, which serves as a propagation medium between the crystal and the specimen to be studied. The ultrasonic pulse is then focussed by the cavity to a point on the specimen surface and then returned to the transducer as in the usual case of ultrasonography. The height of the reflected echo gives a measure of the local impedance and topography so that an acoustic image of the specimen can be formed by mechanically scanning the lens over the specimen's surface. This simple acoustic lens is virtually a perfect focussing element so that the actual resolving power is of the order of the wave length of the acoustic wave in the liquid.
A typical acoustic microscope operates with a resolution in the range of 1 to 10 microns in its most useful frequency range. While this is relatively modest as compared to the resolution of an optical microscope, there are several compensations. Perhaps, the most important is that ultrasonic waves can penetrate materials so subsurface details which are invisible optically can be focussed on. Also, the intrinsic contrast is much higher than in optical microscopy so no special staining techniques are required.
One major drawback of a typical acoustic microscope resides in the lengtht of time required to produce an image of a specimen. Since the ultrasonic waves are focussed on a single point, to provide an image of a given zone of the specimen situated on the surface or in depth of the specimen, the lens must be displaced step by step over the entire zone. This method, takes time and requires some mechanical means to move the lens.
Relatively recently, it has been found that useful information about a specimen can be obtained without necessarily providing an image of the specimen. This technique designated as ultrasonic microspectroscopy broadly consists of scanning the specimen with a lens which is focussed in depth of the specimen, the latter being displaced toward the lens. Only the reflected waves which appear to emanate from the focal point return to the transducer. The most important waves are those that are directly reflected near the center of the lens and a second group incident near the edge of the lens, such that the refracted waves are incident on the specimen surface at the Rayleigh angle. These waves are thus converted into Rayleigh surface waves, where they propagate along the surface and progressively re-radiate into the liquid where they are subsequently transmitted to the transducer. This group of waves interferes with the directly reflected group and it can be easily shown that the phase difference is a function of z, the specimen displacement. For large displacements a series of interference fringes is obtained. The Rayleigh wave velocity can be inferred directly from the fringe spacing and the attenuation from the decrease as a function of z.
The general variation of the power reflection coefficient (the output voltage of the lens' transducer) versus lens to specimen spacing z, often referred as a V(z) curve is characteristic of the specimen and permits to obtain Rayleigh wave velocity and attenuation information.
Most of the prior art devices performing ultrasonic micro-spectroscopy involve a mechanical displacement of the lens or the specimen with the attendant disadvantages (time consuming, high mechanical accuracy required, complicated and expensive signal analysis etc.).